If Proto-Indo-European is the oldest known root language of all modern European languages...

You should also look for information on other domains. I'm very partial to Carl Gustav Jung, You want to read is works to learn about commonalities in human thought, so that you can spot the differences with more ease.
You can start with "Modern Man in Search of a Soul" and "Man and his Symbols". After that, "Symbols of Transformation", "The Archetypes and the Collective Unconscious", and "Answer to Job".

user delivers

Plot twist, old Greek had simple tones. A quick Google search returned this text:
>Yes, the accent in Ancient Greek was tonal, in the sense that it appears that an accented syllable was generally pronounced at a higher pitch than an unaccented one. In this way it resembles the tonal accent of many modern languages, such as most of the Bantu languages of Africa.

OP should also read this article, about a hypothesis on how language vocabulary influences the structure of thought:
en.wikipedia.org/wiki/Linguistic_relativity

It definitely does. I have witnessed it firsthand in Asia. When you have specialized and ever more abstract vocabulary your brain is allowed to think in new ways than it ever could have before.

One of the cool things I learnt from that book is that indo-european has a bunch of borrowed words from some sort of finno-ugric language that probably lived to the north of them. At the same time those finno-ugric peoples also borrowed words from indo-european.

Finally, a disclaimer. I only have high school education, from a Third World country (refer to the post flag). My approach to your question was from a linguistic and psychoanalytic background. Again, I will suggest you to place your question in other boards, so that you can have different perspectives on this subject. Another suggestion is to pay a visit to a state university; buyers beware, specially about wokeism if that is not your thing. But there you surely will get better pointers than at Any Forums. Remember to use thoroughly any sources that can lead you to books: Open Library, Internet Archive (no, these are not the same thing; there is exclusive content in both), Library Genesis, Z Library, yes Reddit (just don't tell anyone), etc.
This is a large subject, and it can lead you to a lifelong research.
Cultural factors, what OP is looking for, are also an influence; language mirrors that. Words act as filters, limiting the inputs you can get. It is recommended to learn a number of other languages (not closely related) to expand your horizons.

What happened so long ago that makes it so difficult for humans to look back and uncover the true beginnings of our species? Maybe it was something so terrible that it really should be left in the past. I wish we had the chance to decide for ourselves.

PIE is not the ancestor of all languages of Europe. We also have Finnish, Estonian and Hungarian, all Finno-Ugric languages, and their ancestor is Proto-Uralic.

As for the root of PIE, the reconstruction can only go so far. It's not a complete fantasy, but it's probably somewhat far from what actually was spoken back then. Some languages ceased to exist without ever acquiring a written form, so the data that could be extracted from them and refine the reconstruction is forever lost.

Consider this: if written Latin never existed, and you only had modern Spanish, French, Portuguese, Italian, Romanian, etc., you wouldn't be able to reconstruct classical Latin from them. The farthest you would get is the language spoken in ~7 AD. Some words would be lost forever; Latin 'equus' (horse) didn't make it into any of the Romance languages. Some grammar structures, likewise, are not reconstructable from that; Latin had 6 cases, most Romance languages, except for Romanian, don't really have any.

With PIE, we have much more languages to work with, but the time period also stretches much further, so imagine the imprecision.

What kind of answer do you want? Remember to not ask questions if you may not like the possible answers. There are some pointers in occultism and also in some more conventional areas.
The short form is decay. In civilization collapses, a lot of information is lost.
There is also the question of languages known to not be related to any other; Basque language, in Europe, comes to mind.
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On things that should not be read on language, beware of relying on the universal grammar theory (refer to the Pirahã language, that contradicts basic propositions of the theory).

I suppose I can imagine that something so terrible happened around the 4k or so years between the older and younger dryas that basic survival took priority over passing on to children such basics as language.

Most likely a sudden rise on sea levels. Even today most people live near shores. I'm not saying something in The Day After Tomorrow style, but something on the scale of decades, making relocation of large numbers of people impossible.
The bad news is that progress to the infinite and beyond is most likely impossible (refer to the Fermi Paradox and the Great Filter).

Indo-European seems to be related to other Siberian languages in a macrofamily called Eurasian.

Uralic and Indo-European are most closely related. The third one would be Uralic or Transeurasian comprised of Mongolic, Turkic, Tungusic and maybe Japanese and Koreanic.

This is somewhat supported by the genetics as all Eurasiatic languages would emerge in the region of ANE presence.